Japanese researchers used supercomputer simulations to reveal how sodium ions form clusters and move within hard carbon anodes, identifying nanopore sizes and transition regions that control diffusion and assess performance in sodium ion batteries. Their findings provide design guidelines for optimizing hard carbon anodes to improve energy density, cycling, and commercialization prospects.
Scientists at Japan’s Institute of Science Tokyo (Science Tokyo) have used supercomputer simulations to unravel the physics of hard carbon (HC) anodes in sodium-ion batteries (NIBs).
HC is a key component in state-of-the-art NIBs, which have received attention in recent years due to the abundance of sodium. As these batteries approach commercialization, researchers are struggling to explain how sodium ions form clusters in HC pores at operating temperatures – and why their overall mobility remains slow
“I believe we are the first group to show the formation of sodium (Na) clusters in nanopores of hard carbon. The diffusion bottleneck for Na ions in hard carbon is also analyzed and visualized at the atomic level for the first time,” said corresponding author Che-an Lin. pv magazine. “We have shown that Na ions have very high diffusivity in most regions in hard carbon, and it is the transition regions between large and narrow graphene interlayer distances that hinder the diffusion of Na ions. This means that if we can further optimize the hard carbon structure, there is an opportunity to significantly improve its speed.”
Lin added that energy density is the main hurdle scientists must overcome before commercialization of NIB can be widespread. “There are a number of companies currently carrying out or planning mass production of Na-ion batteries and selling Na-ion battery products. Most commercial Na-ion batteries focus on fast charging/discharging and wide operating temperature range, which is quite challenging for Li-ion batteries. Therefore, as a complementary technology to Li-ion batteries, Na-ion batteries have shown promising performance,” he said.
Yoshitaka Tateyama, who led the research group, further noted in a statement that “ultimately, widespread adoption of NIBs will increase the total supply of batteries in society, supporting the realization of a carbon-neutral future. By integrating our new insights, our research provides clearer design guidelines for HC materials that can efficiently store sodium, thereby contributing to the development of better NIBs.”
Image: Tokyo Institute of Science
The Tateyama team conducted their research using several powerful supercomputers, including Fugaku, one of the ten fastest systems in the world. On those computers, they performed high-precision density functional theory based molecular dynamics (DFT-MD) simulations, examining different arrangements of sodium ions and graphene sheets.
These simulations revealed that sodium ions in nanopores transition early from a two-dimensional adsorption state to a three-dimensional, quasi-metallic cluster state. Based on the above finding, the team theoretically determined the optimal nanopore diameter for stable sodium storage, which was approximately 1.5 nm.
“Based on our results, we can provide some guidelines for designing HC anode with high plateau capacity and good cycling kinetics,” the researchers said in the article. “To obtain a high plateau capacity, the pore size and pore fraction must be carefully controlled. We show that the optimal pore size is 1.5 nm, and pore sizes smaller or larger than this can lead to an unstable Na cluster. A small range of pore size distribution with an average pore size of ≈1.5 nm should lead to a high plateau capacity.”
Furthermore, the simulation study revealed that certain defect-adsorbed sodium ions, instead of acting as nucleation sites, promote sodium cluster formation by reducing Na–C interactions and the available space for nascent sodium ions in HC nanopores. Furthermore, it has been shown that although sodium ions exhibit locally fast diffusion in well-connected regions of HC, branching or reconnection regions pose serious bottlenecks for ion migration. “These narrower transition regions become clogged by sodium ions until sufficient repulsive force is built up to remove the blockage, creating a rate-limiting step that explains the material’s slow performance,” they explained.
Their findings were presented in “Revealing dominant processes of Na cluster formation and Na ion diffusion in hard carbon nanopores: a DFT-MD study”, published in advanced energy materials.
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